I never said ISO shouldn't be considered the default but if you want to compress then there usually is no harm in that and its not like it should be avoided at all costs but like you said there is alot of variables to consider as well as the CSO and ISO itself. I was using a 16GB MSPD at the time so i'm not sure if that makes any difference.
Also I tested out a 16GB MicroSD in a MSPD adapter by just watching 1 video, boot up MoonShell and listen to music. My findings was that it works just as well as an actual MSPD but when booting up a video, game/app and start up it was a little slower than the MSPD, I would do a speed test but I don't have an app for that.
You use Moonshell for multimedia?
(It's really only "good" on the DS, he's only recently releaed Moonshell for the PSP, for now I'm staying with other homebrews)
In any case, the read/write speeds of adapters are lower however at the right settings the experience may be just as lagless as on normal MSPD's, and considering the price of SD's nowadays I think it's quite a good choice. I'd recommend faster and more expensive MSPD's only to power users and purists.
And yeah, CSO is obviously not "the damned format", it's just that it may cause unexpected issues and if you have the storage space available then you should opt not to use it, especially when it affects overall performance.